One of communication schemes for allowing multiple transmitters/receivers to share a frequency band is carrier sense multiple access with collision avoidance (CSMA/CA) used in wireless LANs. In CSMA/CA, the interference level is measured to detect whether other users are conducting wireless communications prior to starting data transmission. If it is determined that other users are not conducting wireless communication, data transmission is started. If packet collision is detected, transmission is restarted after a time interval determined at random.
FIG. 1 is a schematic block diagram of a CSMA/CA transmitter/receiver 1. The CSMA/CA transmitter/receiver 1 has a receiving antenna, a data signal detector 2 connected to the receiving antenna, and a received power level measuring unit 4 connected to the receiving antenna parallel to the data signal detector 2. The CSMA/CA transmitter/receiver 1 also has a transmission antenna, a transmission availability control unit 6 connected to the transmission antenna and the received power level measuring unit 4, a multiplexer, a data symbol generator 10 connected to the multiplexer 8, and a pilot symbol generator 12 connected to the multiplexer 8 parallel to the data symbol generator 10. The data signal detector 2 outputs information bits of the received signal. Information bits to be transmitted are input to the data symbol generator 10.
The data signal detector 2 detects a data signal received at the receiving antenna to output the received information bits. The received power level measuring unit 4 measures the power level of the received data signal, and supplies the measurement result to the transmission availability control unit 6.
The data symbol generator 10 generates a data symbol sequence based on the information bits to be transmitted, and supplies the generated data symbol sequence to the multiplexer 8. The multiplexer 8 superimposes the data symbol sequence on the pilot symbol sequence generated by and supplied from the pilot symbol generator 12, and supplies the superimposed symbol sequence to the transmission availability control unit 6. The transmission availability control unit 6 allows the superimposed symbol sequence to be transmitted immediately from the transmission antenna if it is determined that other users are not conducting radio communication. If it is determined that other users are conducting radio communication, then the transmission availability control unit 6 allows the superimposed symbol sequence to be transmitted after a prescribed time interval.
However, the above-described conventional technique has some problems. With a CSMA/CA scheme used in wireless LANS, if a user is using a frequency band, the other user cannot conduct radio communication. Consequently, large delay is likely to occur in real-time communications or stream transmission.
In addition, if there are many users simultaneously using the frequency band, an undesirable situation where a certain user cannot communicate for a long time occurs. By restricting interference excessively, the overall communication capacity of the entire system may be reduced.